The State Department announced yesterday that it is pulling some U.S. Embassy officials and all dependents out of Lebanon, and advised Americans who do not have business in the war-torn country to leave as quickly as possible.

The decision followed almost a month of heavy fighting in residential districts of Beirut between Syrian peacekeeping troops and rightist Christian militias.

Some analysts have voiced fears in the past few days that Beirut may be heading for an all-out showdown between Syrian and Christian forces that could plunge Lebanon back into the bloody chaos of the 1976 civil war.

State Department officials said yesterday's decision to reduce the number of Americans in Lebanon was based on any "specific indications" that a major upsurge in the fighting is imminent.

"There hasn't been any dramatic change in the situation," a State Department official said. "Things just haven't been getting any better. The situation has just gotten to the point where this seemed like the reasonable thing to do."

The State Department described to reduce the U.S. Embassy staff in Beirut by five officials - one fifth of the current total - as a "thinning out."

"These are essentially people without whom the embassy can continue to function," an official said.

The department said the dependents who were being sent out of the country consisted of six women - including the wife of Ambassador Richard Parker - and six children.

In advising the fewer than 300 expatriate Americans believed to be living in Beirut to also leave, the U.S. Embassy emphasized that it was "not currently planning to arrange an evacuation" as it did during the 1976 fighting.

But, the announcement said, "The American Embassy in Beirut suggests that American citizens who do not have business in Lebanon should leave at their earliest convenience."

The State Department said the departing U.S. Embassy officials and dependents would fly out of Beirut's international airport on regularly scheduled commercial flights. The five staff members are expected to go initially to Athens.

While most airlines flying out of Beirut report they are fully booked a week in advance, State Department officials said Americans seeking to leave Lebanon should have no trouble getting out of the country.

"A lot of the planes are leaving half empty because of no-shows," an official said.

In addition to the small American expatriate community in Beirut, the State Department said a large number of Lebanese-Americans had also returned to Lebanon following the end of the civil war.

As word of the U.S.Embassy's advisory spread through Beirut, it added to the growing uneasiness over the continuing fighting between the Syrians and Christians.

In a move to halt the violence, Lebanese President Elias Sarkis ordered police and security forces to move into positions between the two sides, and small groups of armed gendarmes began taking up positions yesterday in the downtown area near the main port.

Interior Minister Salah Salman later visited the port, which has been closed since fierce fighting broke out in the area on July 1, and expressed hope that port operations would return to normal today.

But despite the efforts of the security forces, there were no indications that either the Syrian troops or the military forces were abandoning their strongholds.

Heavy Syrian shelling of the capital's Christian residential districts in an effort to tame the militias has thus far claimed an estimated 240 lives this month, nearly all of them civilians.